In the fiber optics field, the term optical bench is commonly used to denote a semiconductor die design particularly adapted for opto-electronic applications. More particularly, an optical bench die is a die having a design which, depending on the components mounted thereon, can serve as the basis for several different optical or opto-electronic applications. For instance, an optical bench die with a laser mounted thereon can be used as an optical transmitter. The same die having no laser mounted thereon, but rather a photodetector, can be used as an optical receiver. One such optical bench die is the silicon optical bench (SiOB) die used as a basic building block in the Laser 2000 series of chips manufactured by Lucent Technologies, Inc. of Murray Hill, N.J. This SiOB die is particularly designed for opto-electronic telecommunication applications, such as in fiber optic telephone networks.
When an optical bench die is used as the basis of a transmitting component, a semiconductor laser diode typically will be mounted on the surface of the die. The current input terminal or terminals of the laser diode are coupled to a signal source. In essence, a semiconductor laser diode converts current into light with the instantaneous light output power level being proportional to the instantaneous value of the current.
In the gain region of the semiconductor laser, current is converted into light and the light is amplified. A portion of the light is allowed to escape through an output facet that is partially reflective and partially transparent. The light which escapes through the facet is the output light of the laser.
For various reasons, it is often desirable to monitor the amount of light generated by the laser responsive to the current applied at the input(s) of the laser. One of the more prevalent reasons for this is the fact that the ratio of the light power generated relative to the applied input current (hereinafter the laser's current-to-light ratio) typically degrades over time. Accordingly, in order to maintain light output levels within required tolerances, it is common to monitor the current-to-light ratio and, through feedback, adjust the input current to the laser in order to maintain light output power within specified tolerances.
One known way to monitor the output light power of the laser diode is to include another partially transparent facet on the rear of the laser and to place a photodetector diode in a position to receive the light output from the rear facet of the laser. As long as the ratio of the amount of light escaping from the rear facet to the amount of light escaping from the front facet (i.e., the relative reflectivities of the front and rear facets) is known, then the amount of light output from the front facet is known by monitoring the light output from the rear facet. The ratio of the reflectivity of the front and rear facets is known as a laser's asymmetry.
The photodetector converts the received light into current. A feedback loop adjusts the current input signal amplification level into the laser as a function of the photodetector output current relative to the input signal current to the laser.
Laser asymmetry can vary widely from device to device under present fabrication technology. For instance, it would not be uncommon for the asymmetry of lasers fabricated in accordance with state of the art technology to vary from ratios of 1:2 to 1:20 from device to device for the same device design fabricated by the same fabrication process in the same fabrication line. In theory, the wide variation in asymmetries is not problematic as long as the asymmetry of the particular device is known and the feedback loop gain is adjusted accordingly. However, in practice, due to power restrictions in devices built using these optical benches, it is highly desirable or even necessary to restrict the current output of the photodetector diodes within very narrow tolerances.
Since, with all other factors remaining the same, wide variations in laser asymmetry will lead to wide variations in photodetector output currents, chip manufacturers have employed various means to restrict photodetector output currents within narrow tolerances. For example, one common technology is active alignment of a photodetector's mounting position relative to the laser position on a die to cause the photodetector's detection face to receive a specified portion of the rear facet output light in order to compensate for laser asymmetry variations. The term active alignment refers to a procedure for aligning the photodetector on the die by powering the laser and photodetector and actively measuring the laser light output power to photodetector current ratio (hereinafter the laser power to photodetector current ratio) as different alignments are tried. When the desired ratio is reached, the photodetector can be bonded to the die in the position that yielded the proper ratio.
While generally yielding accurate placement of photodetectors, active alignment is considerably more time consuming and expensive than passive alignment. The term passive alignment refers to alignment of components for mounting on a die surface based on fiduciary marks on the die. In passive alignment, an automated, robotic machine simply optically locates the fiduciary marks and mounts the given component at a position relative to the mark that is dictated by its programming.
Passive alignment techniques known in the prior art are sometimes unacceptable for placement of a photodetector in view of the problems mentioned above. This is because, in theory, each diode will need to be placed in a different position in order to properly set the laser power to photodetector current ratio.
Particularly, the photodetector can be actively aligned on the die surface so that its detection face receives only a specified portion of the light output by the laser. For instance, using a gold turning mirror and typical die, laser, and photodetector dimensions, optimal alignment of the photodetector will result in about 50% of the light emitted from the rear facet of the laser to reach the photodetector face. Accordingly, if the photodetector is being positioned on a SiOB die which bears a laser having a 1:20 asymmetry ratio, it may be positioned in the optimal position so as to receive 50% of the light output from the rear facet. In order to maintain photodetector output currents of all products within a narrow tolerance, when a photodetector is mounted on a SiOB die upon which a laser with a 1:2 asymmetry has been mounted, the photodetector is actively aligned so as to receive only 5% of the light output from the rear facet. Accordingly, the amount of current generated by the photodetector for a given front facet output light power of the laser is the same for both chips.
In an exemplary active alignment process, the laser diode is permanently mounted to the die surface. Then, the photodetector is temporarily positioned in a first position. The laser diode and the photodetector are then turned on and measurements are taken of the front facet output power of the laser and the corresponding current output of the photodetector to determine whether the photodetector output currents relative to the front facet output power of the laser are within specified tolerances. If not, the photodetector is moved to a new position and a new measurement is taken. The process is repeated until the photodetector output currents are within the specified tolerances. The photodetector is then permanently mounted in the position which yielded photodetector output currents within the specified tolerances. Many other laser/photodiode alignment systems are available which use variations of the active alignment process discussed herein.
The process of active alignment is difficult, expensive and time consuming.
Accordingly, it is an object of the present invention to provide an improved method and apparatus for aligning components for mounting on the surface of a semiconductor die.
It is another object of the present invention to provide an improved method and apparatus for passively aligning a photodetector diode on the surface of an optical bench die.
It is a further object of the present invention to provide an improved method and apparatus for aligning the position of a photodetector on the surface of an optical bench relative to a micro-machined cavity on the bench and a laser diode mounted on the bench.
It is yet one more object of the present invention to provide an improved semiconductor optical bench die adapted to accommodate easy passive alignment of components to be mounted thereon.
It is yet a further object of the present invention to provide an improved method and apparatus for setting the laser diode light output power level to photodetector diode current level ratio on a populated optical die.